ABSTRACT
In the mid-1990s, studies using the atomic force microscope (AFM) on live pancreatic acinar cells demonstrated for the irst time the presence of 100-180 nm in size cup-shaped lipoprotein secretory portals called “porosomes” at the cell plasma membrane. Subsequent studies using both AFM and electron microscopy (EM) conirmed the presence of porosomes in all secretory cells, including beta cells of the endocrine pancreas, growth hormone-secreting cells of the pituitary gland, chromafin cells, hair cells of the inner ear, astrocytes, and neurons. While porosomes measure 100-180 nm in the exocrine pancreas and in endocrine and neuroendocrine cells, porosomes in neurons and astrocytes are much smaller, measuring just 10-17 nm. AFM and EM studies further demonstrate the docking and transient fusion (kiss-and-run) of secretory vesicles at the porosome base via SNAREs and calcium. v-SNARE present in secretory vesicle membrane, and t-SNAREs at the porosome base interact in a circular array to establish continuity or fusion pore
formation between the opposing bilayers. The size of the fusion pore formed by the t-/v-SNARE ring complex is dependent on the curvature of the secretory vesicle and the porosome base. To enable the expulsion of intravesicular contents through the porosome, secretory vesicles docked at the porosome base undergo volume increase primarily via water channels or aquaporins (AQPs), resulting in an increae in intravesicular pressure. The greater the intravesicular pressure, the greater is the amount of secretion. These indings established in the past 16 years clarify our understanding of the generation of partially empty secretory vesicles in cells following secretion, as opposed to the complete merger of the secretory vesicle membrane at the cell plasma membrane. The porosome discovery has resulted in a paradigm shift in our understanding of the secretory process in cells. It now makes sense why a fast secretory cell like the neuron would require neurotransmitter transporters at the synaptic vesicles membrane, to be able to rapidly replenish spent synaptic vesicles following neurotransmitter release. It seemed illogical to believe that in mammalian cells, secretory vesicles completely merge at the cell plasma membrane, while single cell organisms and bacteria all have well developed secretory machinery for the precise delivery of secretory products. In this chapter, the structure, dynamics, isolation, reconstitution, and composition of the porosome-the universal secretory portal at the cell plasma membrane-are briely summarized.
